The present invention relates to a brake valve mechanism in which the movement of an actuating lever displaces a control piston to apply pressure or to release pressure applied to a brake cylinder. More specifically, the present invention relates to a parking brake valve which applies pressure and releases pressure from a spring loaded braking cylinder.
Hydraulic external force brake systems are power brake systems in which the pressure applied to the brakes comes from a source of a pressure medium or a hydraulic fluid such as an accumulator or a pump, for example. Parking brake valves in such systems typically cooperate with spring loaded brake cylinders which provide the energy necessary for braking. The brake is released by increasing the pressure in a line leading from a brake connection on the valve to the spring loaded cylinder.
The Scandinavian countries require that parking brakes operate in the following manner: (1) When the vehicle is braked, i.e., when the line leading to the spring loaded cylinder is at a low pressure, the lever which actuates the parking brake in response to a foot or hand motion must be in a defined braking position. (2) In case of a breakdown, such as the rupture of a line in the parking brake system, the vehicle can only be operated if the parking brake valve is in its braking position, and the actuating lever must also be in its defined braking position.
One known solution which satisfies the second requirement is to prevent the starting of the vehicle by using a pressure switch together with a device for monitoring the position of the actuating lever. In particular, when a breakdown occurs, the actuating lever must be monitored to determine whether it is in the braking position.
It would be advantageous to provide a brake valve system which would meet these requirements indicated above under items (1) and (2) hydraulically rather than electrically and without preventing starting of the vehicle engine.